1. Field of Invention
The present invention relates to light emitting diodes (LEDs) and, more particularly, to a phosphor-converted LED device that utilizes one or more phosphor thin films for converting primary light emitted by the LED into one or more other frequencies of light in order to generate white light.
2. Description of Related Art
With the development of efficient LEDs that emit blue or ultraviolet (UV) light, it has become feasible to produce LEDs that generate white light through phosphor conversion of a portion of the primary emission of the LED to longer wavelengths. Conversion of primary emission of the LED to longer wavelengths is commonly referred to as down-conversion of the primary emission. An unconverted portion of the primary emission combines with the light of longer wavelength to produce white light. LEDs that produce white light are useful for signaling and/or illumination purposes.
Currently, state-of-the-art phosphor conversion of a portion of the primary emission of the LED is attained by placing a phosphor in an epoxy that is used to fill the reflector cup, which houses the LED within the LED lamp. The phosphor is in the form of a powder that is mixed into the epoxy prior to curing the epoxy. The uncured epoxy slurry containing the phosphor powder is then deposited onto the LED and is subsequently cured.
The phosphor particles within the cured epoxy generally are randomly oriented and interspersed throughout the epoxy. A portion of the primary light emitted by the LED passes through the epoxy without impinging on the phosphor particles, whereas a portion of the primary light emitted by the LED impinges on the phosphor particles, thereby causing the phosphor particles to emit complimentary light. The combination of the primary blue light and the phosphor-emitted light produces white light. One disadvantage of utilizing the epoxy comprising the phosphor particles is that uniformity in the white light emitted by the LED is difficult, if not impossible, to obtain. This non-uniformity is caused by non-uniformity in the sizes of the phosphor particles mixed into the epoxy slurry. Currently, phosphor powders having uniform phosphor particle sizes generally are not available. When the phosphor powder is mixed into the epoxy slurry, the larger phosphor particles sink faster than the smaller phosphor particles. This non-uniformity in spatial distribution of the phosphor particles exists in the epoxy once it has been cured.
Therefore, obtaining a uniform distribution of the phosphor particles within the epoxy is very difficult, if not impossible, due to the non-uniformity of the sizes of the phosphor particles. This inability to control the sizes of the phosphor particles and their locations within the epoxy results in difficulties in producing LED lamps that emit white light in a consistent manner. Therefore, the quality of the white light produced by LED lamps may vary from one lamp to another, even for a given model manufactured by a particular manufacturer.
Attempts have been made to overcome the disadvantages of using phosphor powders mixed with epoxies by placing luminescent organic dye films on a lens that encases the LED. The dye is carefully positioned on the lens at a particular location so that it totally absorbs all of the primary light impinging thereon and converts the primary light to light of a longer wavelength. A fraction of the primary light emitted passes through the lens without impinging on the dye. The primary light that does not impinge on the dye then combines with the longer-wavelength light to produce white light. Since the dye totally absorbs the primary light impinging thereon, any variation in the fraction of the primary light that is summed with the longer-wavelength light is supposed to be eliminated.
However, this latter approach also has several disadvantages. The placement of the dye on the lens is subject to manufacturing uncertainties, which may result in variations in the white light produced. Also, dyes that are stable over long periods of time generally are not available. As a result, wide spread use of wavelength-converting dyes has not occurred.
Accordingly, a need exists for a phosphor-converted LED that overcomes these problems and disadvantages.